class_docstr: Add the `DefaultTokens.MASK_BEFORE` placeholder between
  the prompt and the response in an LM finetuning exemple.
  This is necessary to enable the 'zero-out prompt loss' mechanism.
code: def __init__(self, opts):
      super().__init__(opts)
  
file_path: OpenNMT-py/onmt/transforms/insert_mask_before_placeholder.py
imports: from onmt.constants import DefaultTokens
  from onmt.transforms import register_transform
  from onmt.utils.logging import logger
  from .transform import Transform
label_desc: Use image node_id 1 to create a new InsertMaskBeforePlaceholdersTransform object from inherited base classes: Transform with example: obj = InsertMaskBeforePlaceholdersTransform(opts)